With the rapid progress of information technology and the rapid development of the high-tech industry, most of the sophisticated electronic instruments and equipment rely on high-quality power supply to maintain a normal operation. Among a variety of power-supplying solutions, uninterruptible power supply can ensure a nonstop and a high-quality power supply. Therefore, uninterruptible power supply has become the best solution for providing a high-quality power supply. Because different uninterruptible power supplies have different power conversion efficiency, the use of uninterruptible power supply will pose a 10%-30% surcharge on the utilities every year.
Referring to FIG. 1, the circuitry of a conventional uninterruptible power supply is shown. As shown in FIG. 1, the uninterruptible power supply 1 includes an AC/DC converter 11, a charger circuit 12, a battery module 13, a DC/DC converter 14, an inverter 15, a controller 16, a static transfer switch 17 and a bypass route 18. The function and the association of each circuit elements of the uninterruptible power supply 1 are described as follows.
When the input power source Vin is supplying power normally, the controller 15 will manipulate the AC/DC converter 11 to convert the input AC voltage Vin into a DC voltage having a predetermined voltage level and provide this DC voltage for the charger circuit 12 and the inverter 15. In the meantime, the controller 16 will manipulate the inverter 15 to convert this DC voltage into a standard and reliable AC voltage. The output AC voltage V1 of the inverter 15 is provided for the load 19 through the static transfer switch 17 (where the load voltage Vout is the output AC voltage V1 of the inverter 15). In the meantime, the charger circuit 12 will convert the DC voltage outputted from the AC/DC converter 11 into a DC voltage tailored to charge the battery module 13.
When the input power source Vin is unavailable for supplying power due to blackout or brownout and thereby causing the degradation of power, the controller 16 will manipulate the DC/DC converter 14 to convert the DC voltage outputted from the battery module 13 into a DC voltage required by the inverter 15. Next, the inverter 15 will convert the DC voltage outputted from the DC/DC converter 14 into an AC voltage, which is provided for the load 19 through the static transfer switch 17. In this case, the power used by the load 19 is supplied by the battery module 13 that is formed by a plurality of batteries. More specifically, the duration of the battery module 13 for sustaining supplying power is dependent on the number of batteries of the battery module 13.
When the input power source Vin is supplying power normally, the AC/DC converter 11 of the uninterruptible power supply 1 will convert the input AC voltage Vin into a DC power, and then the inverter 15 will convert the DC power into a standard and reliable AC power. The output AC power V1 of the inverter 15 is provided for the load 15 through the static transfer switch 17. Because the AC/DC converter 11 and the inverter 15 will output energy when the input power source Vin in supplying power normally, the power conversion efficiency of the AC/DC converter 11 and the inverter 15 will lower the power utilization.
In order to improve the power utilization, another operating method for the uninterruptible power supply is proposed. This operating method is carried out in a manner that when the input power source Vin is supplying power normally, the controller 16 will manipulate the static transfer switch 17 to provide the input AC power Vin for the load 19. That is, when the whole system is normal, the input AC power Vin is provided for the load 19 through the bypass route 18. In the meantime, the inverter 15 will perform power conversion process to output AC power through the static transfer switch 17, and the peak voltage of the output voltage V1 of the inverter 15 is a predetermined rated peak voltage Vp1. In the meantime, the charger circuit 12 will convert the DC voltage outputted from the AC/DC converter 11 into a DC voltage tailored to charge the battery module 13.
When the peak voltage or the frequency of the input voltage Vin is abnormal, for example, when the peak voltage is increased or decreased by 10% of the rated peak voltage or when the frequency is increased or decreased by 5% of the rated frequency, the controller 16 will manipulate the static transfer switch 17 to provide the output voltage V1 of the inverter 15 for the load 19 through the static transfer switch 17 (where the load voltage Vout is the output AC voltage V1 of the inverter 15), thereby enhancing power utilization. The uninterruptible power supply employing such control method is called economic-mode uninterruptible power supply, or ECO-mode uninterruptible power supply. Nevertheless, such control method and control configuration is applicable to non-inductive load and non-motorized load. In the case of inductive load and motorized load, when the peak voltage or frequency of the input voltage Vin is abnormal and the static transfer switch 17 switches the power delivery route to provide the output voltage V1 of the inverter 15 for the load 19, the phase difference between the input voltage Vin and the output voltage V1 of the inverter 15 would be large. When the phase difference between the input voltage Vin and the output voltage V1 of the inverter 15 is sufficiently large, for example, above 20 degrees, the uninterruptible power supply would perform asynchronous power conversion and cause inrush current. This would even burn down the uninterruptible power supply and inhibit the uninterruptible power supply from supplying power to the load 19.
Referring to FIG. 2, the waveform diagram showing the voltage waveforms and current waveforms associated with the uninterruptible power supply. As indicated in FIG. 2, the peak voltage, frequency or phase of the input voltage Vin is abnormal at time t1. In the meantime, the controller 16 will manipulate the static transfer switch 17 to switch the power delivery route to provide the output voltage V1 of the inverter 15 for the load 19. However, the phase difference between the input voltage Vin and the output voltage V1 of the inverter 15 is quite large, for example, above 20 degrees, so that a large voltage difference is generated between the load voltage Vout and the output voltage V1 of the inverter 15. Under this condition, the static transfer switch will switch the power delivery route so as to cause a large inrush current on the load current lout. This would even burn down the uninterruptible power supply and inhibit the uninterruptible power supply from supplying power to the load 19.
Hence, it is urgent for those skilled in the art to develop a control method for use by an uninterruptible power supply to remove the above-mentioned drawbacks encountered by the prior art.